The present invention relates to an iron core assembly for magnetizing permanent magnets for use in magnetic-brush type electrostatic developing apparatus and, more particularly, to an iron core assembly for imparting a plurality of axially extending magnetic poles on the cylindrical surface of a columnar magnet which is generally referred to as "magnetic roll."
Developers conventionally used in electrostatic developing apparatus for visualizing an electrostatic latent image are divided into two types of bicomponent developer consisting of a magnetic carrier and toner particles, and monocomponent developer having magnetic particles contained by toner particles.
Also, there have been two ways of developing latent image by these developers: cascade type method and magnetic-brush type method. In the past, the cascade type method had been commonly used. However, recently, the magnetic-brush type method has been getting popular, because of the so-called edge effect which causes an insufficient development at the central portion of the region to be developed and other drawbacks inherent in the cascade type method.
In the magnetic-brush type developing apparatus, developer particles are conveyed to a zone close to a latent image held on an electrostatic image carrier, in accordance with a rotation of developing roll. A brush-shaped protrusion is a formation of the developer particles on the surface of the developing roll, due to the magnetic force of a permanent magnet incorporated in the developing roll. The developer particles are transferred to the electrostatic latent image to visualize the latter, as the image is rubbed by the brush-like protrusion of the developer particles, in accordance with the rotation of the developing roll or the movement of the latent image itself.
There have been proposed a variety of types of permanent magnets for the developing rolls of the kind described.
For example, the specification of U.S. Pat. No. 3,455,276 (Anderson) discloses a columnar permanent magnet consisting of a plurality of previously magnetized sector-shaped rubber permanent magnets adhered to the cylindrical surface of a rotary shaft, while the specification of U.S. Pat. No. 3,402,698 of Kojima et al discloses a developing roll constituted by block-shaped permanent magnets.
Also, it has been acknowledged that the permanent magnet for use in the developing roll is most advantageously made of unitarily formed ceramic permanent magnets. It will be seen that a uniform magnetic flux distribution is obtained along the entire length of an axially extending unitary ceramic permanent magnet, because there is no discontinuity of magnet pieces on the magnetic poles. Specifications of U.S. Pat. Nos. 3,828,730 and 3,952,701 of Yamashita et al exemplarily discloses developing rolls making use of the unitary ceramic permanent magnet.
It is quite difficult to magnetize the unitary ceramic permanent magnet piece. Only Japanese Utility Model Laid-open Publication No. 14800/1976 (Published on Feb. 3, 1976) teaches a broad concept of such a magnetization. However, even this Publication fails to disclose a concrete method of magnetization. Thus, practical magnetizing means for imparting a complicated magnetic pattern to a columnar magnet have not been proposed up to now.
It is therefore an object of the invention to provide magnetizing means capable of imparting an extremely strong attracting force to a developing roll.
More specifically, the invention aims to provide magnetizing means capable of magnetizing a columnar permanent magnet material in such a manner that the magnetized columnar permanent magnet exhibits a rectangular-wave-like pattern of magnetic flux density distribution along the circumference thereof, so that the magnetic flux density may abruptly increase and decrease over the circumference of the magnet, thereby to exert a large attracting force on the magnetic developer particles.
To this end, according to the invention, there is provided an iron core assembly for magnetizing a columnar permanent magnet for use in an electrostatic developing apparatus, comprising:
a plurality of axially extending and circumferentially disposed magnetic poles defining at their radially inner ends a through-bore for receiving a columnar permanent material to be magnetized, said inner ends being adapted to be located in the close proximity of the outer cylindrical surface of said columnar permanent magnet material received by said through-bore, and
coil windings for magnetizing said magnetic poles, said coil windings being wound around said magnetic poles and received by grooves formed between adjacent magnetic poles,
wherein the circumferential width of said magnetic pole is greater than that of said groove.
The above and other objects, as well as advantageous features of the invention will be understood from the following description of the preferred embodiments taken in conjunction with the attached drawings.